Molecular pump comprising a pump cylinder equipped with a thread

ABSTRACT

This invention relates to an improvement in a molecular pump comprising a pump cylinder provided with a thread and a structural element rotating therein, the improvement comprising prefabricated helical springs, having selective profile crosssections, held at the wall of the molecular pump cylinder, THE CONTACT SURFACES OF THE PROFILE CROSS-SECTIONS FACING THE CYLINDER WALL BEING SUFFICIENTLY WIDE THAT THE LINE OF SEPARATION BETWEEN THE COURSES OF THREAD AND THE WALL WITH RESPECT TO THE FREE LENGTH OF PATH OF GAS MOLECULES IS TIGHT.

finite States Patent Gehring et a1.

[ 1 MOLECULAR PUMP COMPRISING A PUMP CYLINDER EQUIPPED WITH A THREAD [75] Inventors: Fritz Gehring, Daisendorf; Karl Jager, Lindau-Schachen, both of Germany [73] Assignee: Dornier A.G.,

- Friedrichshafen/Bodensee,

Germany 221 Filed: Mar. 27, 1972 21 Appl. No.: 238,231

[30] Foreign ApplicationPriority Data Apr. 17, 1971 Germany 2118738 [52] US. Cl 415/72; 415/90, 416/176 [51] Int. Cl. F04d- 5/00 [58] Field of Search 415/90,. 71-75;

[5 6] References Cited UNITED STATES PATENTS 1,065,208 6/1913 Brauer i. 415/75 [451 Apr. 9, 1974 1,492,846 5/1974 Holweck 415/72 2,730,297 1/1956 Van Dorsten et a1. 415/72 FOREIGN PATENTS OR APPLICATIONS 1,010,235 6/1957 Germany 415/71 Primary Examiner-C. J. l-lusar Attorney, Agent, or FirmJames E. Bryan, Esq.

[ 5 7 1 ABSTRACT This invention relates to an improvement in a molecular pump comprising a pump cylinder providedwith a thread and a structural element rotating therein, the

improvement comprising prefabricated helical springs,-

having selective profile cross-sections, held at the wall of the molecular pump cylinder,

the contact surfaces of the profile cross-sections facing the cylinder wall being sufficiently wide that the. line of separation between the courses of thread and the wall with respect to the free length of path of gas molecules is tight.

7 Claims, 3 Drawing Figures PATENTEBAPR '9 lQM SHEEI 1 U? 2 PATENTEUAPR 91914 3,802,794

sum 2 BF 2 FIG. 3

MOLECULAR PUMP COMPRISING A PUMP CYLENDER EQUIPPED WITH A THREAD The present invention relates to a molecular pump comprising a pump cylinder provided with a thread and a structural part rotating therein, such as is used for feeding or conveying gases, for example in gas ultracentrifuges.

Known constructions of molecular pumps are made as separate threaded bushings and are inserted into the container of gas ultracentrifuges, or the molecular pump thread is integrated directly with the container. The construction of such molecular pumps is determined by the required pressure ratios in the centrifuge and depends upon the gases to be conveyed. Accordingly, they are made as single-thread or multiple-thread internally threaded bushings having amore or less significant pitch and screw height, as well as generally with a square thread profile. For purposes of making molecular pumps, the processes of cutting and noncutting deformation or shaping as well as casting and injection molding are employed.

The disadvantage of these conventional prior art constructions is that the methods of manufacture employed therefor are relatively expensive, and an exchange of molecular pumps having a different number of threads and different length without replacing the container and the thread bushing is not possible.

, It is the object of the present invention to manufacture molecular pumps by means of a relatively simple and cost-saving use of cutting or non-cutting deformation or shaping and to thereby attain at the same time the exchangeability of the molecular pump threads.

This object is attained, according to the present invention, by virtue of the fact that inserted in the molecular pump cylinder are prefabricated helical springs with a selective profile cross-section which are held at the wall of the molecular pump cylinder. The contact surfaces of the profile cross-section of the helical springs which face the molecular pump cylindrical wall and are made to fit are so wide that the separating line between the courses of the thread and the wall must be considered as tight or sealing withrespect to the free length of the path of the gas molecules.

In a further embodiment of the present invention, th helical springs are wound in multiple threads and like ones are telescopically inserted within each other. Furare further elongated with theaid of a device and slid into a production sleeve, being equipped with the diameter-zero measure of the container, and also of the molecular pump cylinder, and relieved. After removal of p the device, the inside diameter of the helical springs is brought to the diameter required. With the aid of the same device, the helical springs are again taken out of the production sleeve and inserted directly into'the molecular pump cylinder or into the container. When a molecular pump cylinder is employed, it may be identical with the production sleeve.

Moreover, the processing of the inside diameter of the helical springs also may be eliminated if the required tolerances are relatively great, or if, starting from the manufacture of the profile material including the winding of the springs up to the device, the smallest tolerances are maintained. The selective profile crosssection of the inserted helical springs is preferably made either square, trapezoidal, triangular, or in a circular fashion. By virtue of the processing of the outside diameter of the helical springs and with the correct pitch, the springs will rest flushagainst the wall of the molecular pump cylinder. The interposed separating line is to be considered as tight because the free length of the path of the gas molecules in the vacuum is very great and the latter therefore cannot travel along the contact or abutment surfaces from one course of thread into the other.

A further embodiment of the present invention includes the series connection of several helical spring unitswith respectively different pitch in a sequence thermore, the helical springs are held at the wall of the molecular pump cylinder by virtue of the radiallyacting initial spring tension. Furthermore, the side of the profile cross-section of the prefabricated helical springs facing the wall extends parallel to the wall.

The advantage of such a construction and provision of molecular pumps, as compared to the conventional pumps, is that helical springs are wound from a suitable material by means of a device and have initially a lower pitch than is normally true for molecular pumps. Depending upon and in accordance with the required number of threads for the molecular pump, these helical springs are turned within each other, fixed in a device in equal intervals, and stretched or elongated until the required pitch of the molecular pump is obtained. The helical spring thus produced is turned off at the outside thereof, in other words at the subsequent contact or abutment surface or area, to the tolerance fit of the inside diameter of the container and also the pump cylinder. Thereafter, the spiral or helical springs adapted to be selected, whereby a step molecularpump' is formed in the container or in a common molecular pump cylinder. The ends of the helical springs are advantageously bent-off in the axial direction.-

The selective embodiments of profile cross-sections on molecular pumps as well as the series connectionof several helical springs afford the advantage that they can be individually designed as molecular pumps depending upon the respectively required conditions, whereby it is possible to control the effect thereof in an optimum manner. The bending-off of the spring ends has the further advantage from the standpoint of manufacturing techniques that the helical springs are fas tened and secured in position in the device and that a twisting or slipping is therefore rendered impossible.

One embodiment according to the present invention will now befurther described hereinafterand explained on the basis of the accompanying drawing, wherein F IG. 1 illustrates a longitudinal cross-section through a five-thread molecular pump,

FIG. 2 illustrates the molecular pump of FIG. 1 in a top plan view thereof, and

FIG. 3 schematically illustrates a molecular pump with several helical spring units having respectively different pitches.

The molecular pump 1' shown in FIG. 1 in a longitudinal cross-section has a five-threaded thread 2 having a square cross-section and profile 3. The thread 2 proper consists of the helical springs 4, 5, 6, 7 and 8 (see also FIG. 2) which are wound, at a relatively low pitch, in a five-threaded manner and telescopically inserted within each other. They are pressed against the wall 9 of the container (which has been merely indicated in the figure) due to the radially acting initial spring stress thereof, and maintained in this position. When small tolerances are maintained during the processing of the 3 outside diameter D of the helical springs and with a correctly predetermined pitch S, the spring profiles 3 will rest flushwith the widecontact or bearing surfaces lflthereof in a manner such that the separating line between the contact or bearing surfaces 10, the threaded and helical springs 4, 5, 6, 7 and 8 and the wall 9 of the container isto be considered as tight. Since the free path length of the gas molecules in the vacuum prevailing in the container is very large, they cannot pass along the contact or bearing surface 10 from one thread and helical spring course into the other.

By virtue of theloose shape of the helical springs 4, 5, 6, 7 and 8 and-the relatively simple way of making the same, a direct exchange of these springs within the container or the entire threaded bushing is possible (not shown in the drawing). Moreover, the result thereof will be a possibility of varying the spring protiles 3 as well as the pitch S of the helical springs 4, 5, 6, 7 and 8. If required, the expansion to a step molecular pump can be carreid out by a series connection of several helical springs 4, 5, 6, 7, and 8. v

The ends 11 of the helical springs 4, 5, 6, '7 and 8 which are bent-off in the axial direction render possible, during the manufacturing operation, a fastening and a securing in position thereof within the device. Any slipping or twisting is thereby precluded.

Apparent from FIG. 2 is the molecular pump 1 in a top plan view with the individual five-threaded helical springs 4, 5, 6, 7 and 8, which are in each case arranged offset about 7 2. 7

FIG. 3 schematically illustrates a molecular pump 1 in'which the arrangement of several helical spring units 4, S, 6, 7 and 8, with respectively different pitches is apparent.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is: I

1. In a molecular pump comprising a pump cylinder provided with a thread and a structural element rotat ing therein, the improvement which comprises prefabricated helical springs, having selective profile crosssections, held at the wall of the molecular pump cylinder,

. the contact surfaces of the profile cross-sections facing the cylinder wall being sufficiently wide that the line of separation between the courses of thread and the wall with respect to the free length of path of gas molecules is tight.

2. A molecular pump according to claim 1 in which the springs are multiple chreaded wound helical springs.

3. A molecular pump according to claim 2 including similar helical springs telescopically inserted within each other. i

4. A molecular pump according to claim 1 in which the helical springs are held at the wall of the molecular pump cylinder by radially-acting initial spring stress.

5. A molecular pump according to claim 1 in which the side of the 'profile cross-sections of the prefabricated helical springs facing the wall extend parallel to a sequence adaptedto be selected, connected in series.

7. A molecular pump according to claim 1 in which the ends of the helical springs are bent-off in the axial direction. 

1. In a molecular pump comprising a pump cylinder provided with a thread and a structural element rotating therein, the improvement which comprises prefabricated helical springs, having selective profile cross-sections, held at the wall of the molecular pump cylinder, the contact surfaces of the profile cross-sections facing the cylinder wall being sufficiently wide that the line of separation between the courses of thread and the wall with respect to the free length of path of gas molecules is tight.
 2. A molecular pump according to claim 1 in which the springs are multiple-threaded wound helical springs.
 3. A molecular pump according to claim 2 including similar helical springs telescopically inserted within each other.
 4. A molecular pump according to claim 1 in which the helical springs are held at the wall of the molecular pump cylinder by radially-acting initial spring stress.
 5. A molecular pump according to claim 1 in which the side of the profile cross-sections of the prefabricated helical springs facing the wall extend parallel to the wall.
 6. A molecular pump according to claim 1 including a plurality of helical spring units with varying pitch at a sequence adapted to be selected, connected in series.
 7. A molecular pump according to claim 1 in which the ends of the helical springs are bent-off in the axial direction. 